Until recent times, the technology of glow plugs, as applied to diesel internal combustion engines, has primarily evolved to satisfy the requirement of merely assisting the startup of such engines. In this application, it is understood that the diesel engines are burning autoignitable fuels.
Such conventional glow plugs are designed to be temporarily energized, by electrical-resistance heating, to a preselected moderately high temperature (for example, about 900.degree. C./1650.degree. F.) only during the brief period of starting. When cranking the engine during startup, atomized fuel sprayed from an injector contacts or passes in close proximity to the hot glow plug and ignition of the fuel is effected primarily by surface ignition. Because the rotational speed of the engine is quite slow during the cranking and startup phase, fuel remains in the vicinity of the glow plug for a relatively long time compared with normal engine operation. Consequently, the ignition of conventional fuel in a relatively cold engine is accomplished even at the above moderately high temperature. Once the engine is started, such glow plugs are deenergized and the engine continues to operate solely by autoignition of the fuel. Consequently, the deenergized glow plugs are allowed to cool down to a lower temperature which is approximately the engine mean cycle temperature (for example, about 675.degree. C./1250.degree. F.) during normal engine operation.
It has also been customary to preheat conventional glow plugs to the moderately elevated temperature prior to cranking and starting of the diesel engine. In commercial vehicles, such as earthmoving tractors or heavy-duty trucks, there used to be little concern about the time required (typically about one to two minutes) for preheating the glow plugs to the moderately elevated temperature. However, the increased application of diesel engines to light-duty trucks and passenger cars in recent years has caused a greater demand on being able to preheat the glow plugs in a much shorter period of time (typically about one to two seconds being considered acceptable). Thus, in recent years, the technological development of glow plugs has also focused on providing temporarily energizable glow plugs which require less time to preheat before the engine is cranked and started.
In response to scarce and dwindling supplies of conventional diesel fuel as well as the environmental need to develop cleaner burning engines, manufacturers have been developing engines which are capable of burning alternative fuels such as methanol, ethanol, and various gaseous fuels. However, such alternative fuels typically have a relatively low cetane number, compared to diesel fuel, and therefore are reluctant to ignite by mere contact with the heat of compressed intake air.
Applicants have been early leaders in the development of ignition-assisted engines which operate on the diesel cycle but which differ from conventional diesel or compression-ignition engines in that the ignition of the injected fuel and propagation of the flame is not effected primarily by the fuel contacting the heat of compressed intake air during normal engine operation. This hybrid type of engine having ignition-assist will hereinafter be generally referred to as a diesel-cycle engine.
As shown in U.S. Pat. No. 4,721,081 issued to Krauja et al. on Jan. 26, 1988 and U.S. Pat. No. 4,548,172 issued to Bailey on Oct. 22, 1985, one way of facilitating ignition of such fuels is to provide an ignition-assist device which extends directly into the engine combustion chamber. For example, the ignition-assist device may include a continuously energized glow plug which is required to operate at a very high preselected temperature throughout engine operation. For example, such very high preselected temperature may be about 1200.degree. C./2192.degree. F. in order to ignite the above mentioned alternative fuels.
Applicants initially tried to use conventional glow plugs in this application. One type of conventional glow plug is generally shown in U.S. Pat. No. 4,476,378 issued to Takizawa et al. on Oct. 9, 1984. This glow plug has a heating element assembly consisting of a wire filament wound as a single helix around a mandrel which is positioned in a blind bore of a sheath. The sheath is made of heat resistant metal such as stainless steel. The remaining space in the blind bore is then filled with a heat resistant electric insulating powder such as magnesia. In order to compress the heat resisting electrically insulating powder tightly around the filament for providing adequate support of the filament wire and for effecting adequate heat transfer to the metal sheath, the sheath is normally swaged inward to decrease its inside diameter and thereby compact the powder. One end of the filament at the bottom of the blind bore is connected to the metal sheath so that the metal sheath forms part of the electrical circuit.
Applicants found that a glow plug sheath formed from commercially feasible metallic materials is too vulnerable to oxidation and corrosion attack if it is continuously heated in the and exposed to an engine combustion chamber. The sheath is severely attacked by impurities, such as sodium, sulfur, phosphorus and/or vanadium, which enter the combustion chamber by way of fuel, lubrication oil, ocean spray and/or road salt. The metallic sheath is eaten away by these impurities so that the wire filament becomes exposed. The exposed wire filament is then subject to oxidation and corrosion attack and quickly fails.
Another type of conventional glow plug is generally shown in U.S. Pat. No. 4,502,430 issued to Yokoi et al. on Mar. 5, 1985. In this glow plug, the heating element assembly has a spirally-wound wire filament formed from tungsten or molybdenum which is bent in a generally U-shape. The wire filament is embedded in a ceramic insulator formed from silicon nitride (Si.sub.3 N.sub.4). This design is advantageous for the construction of a ceramic glow plug not only because this ceramic material is an electrical insulator but also because this material can be hot pressed to effect good heat transfer from the filament to the ceramic material. In addition, silicon nitride possesses appropriate physical properties such as high strength, low coefficient of thermal expansion, high Weibull modulus and high toughness to permit the glow plug tip to survive the severe thermal and mechanical loadings imposed by the engine cylinder.
This glow plug design exhibits satisfactory life when the heating element assembly is electrically energized only during engine startup to effect ignition of the fuel in a conventional diesel engine. However, Applicants have found that this heating element assembly exhibits an unacceptably short life, for example about 250 hours, when operated continuously to effect ignition of methanol fuel in diesel-cycle engines operating in highway trucks. Similar to the metallic sheaths discussed above, the hot surface of the silicon nitride heating element assembly is vulnerable to severe oxidation and corrosion attack from impurities such as sodium, vanadium, phosphorus and/or sulfur. The silicon nitride covering is eaten away by these impurities so that the wire filament becomes exposed. The exposed wire filament is then subject to oxidation and corrosion attack and quickly fails.
Another type of known glow plug is disclosed in U.S. Pat. No. 4,786,781 issued to Nozaki et al. on Nov. 22, 1988. In this arrangement, a heating element has a generally U-shaped tungsten filament embedded in a silicon nitride insulator similar to that shown in Yokai et al.. However, the silicon nitride insulator is then covered, using a process called chemical vapor deposition, with a coating of highly heat and corrosion resistant material, such as alumina (Al.sub.2 O.sub.3), silicon carbide (SiC) or silicon nitride (Si.sub.3 N.sub.4) in an attempt to minimize erosion and corrosion due to combustion gases.
While this reference avers that the coating adequately protects the filament and silicon nitride covering shown in this glow plug against oxidation and corrosion attack, it has been Applicants' experience that ceramic coatings typically exhibit durability problems when they are applied to a glow plug heating element assembly which is continuously energized at a high temperature. If the coating is applied as a relatively thin layer, the coating quickly disappears from the heating element assembly due to the effects of corrosion and erosion. On the other hand, if the coating is applied as a relatively thick layer, the coating quickly flakes off the heating element assembly. Applicants believe such failure is caused primarily by unacceptably high thermal stresses, that are induced in the thick coating, as well as insufficient bonding of the coating to the insulator.
The present invention is directed to overcoming one or more of the problems as set forth above.